That cannot be done without function.

Code:

---

bmap sitbutton ="sitbutton.bmp";
bmap sit = "sit.bmp";

panel panel1
{
  bmap = sit;
  pos_x = 70;
  pos_y = 600;
  layer = 1;
  flags = visible,transparent;
  on_click = change_sitbtn;
}

function change_sitbtn(panel)
{
  if(panel.bmap == sit)
  {
    panel.bmap = sitbutton;
  }
  else
  {
    panel.bmap = sit;
  }
}

---

NOT TESTED